Raman amplification system utilizing modulated second-order raman pumping

ABSTRACT

Higher-order Raman pumping is combined with pump modulation in a Raman amplified optical communication system. First- and second-order Raman pumps are both modulated and launched in opposite direction to signals in an optical fiber. Relative timing of the pumps is controlled to optimize lateral signal power distribution along the fiber.

TECHNICAL FIELD

The present invention relates to a Raman amplified optical communicationsystem utilizing higher-order Raman amplification with modulated Ramanpumps.

BACKGROUND ART

State-of-the-art wavelength division multiplexed (WDM) optical fibertransmission systems employ distributed Raman amplification (DRA) inaddition to discrete amplifiers. DRA partly compensates fiber lossesalong the transmission fiber and thus allows increasing the distancebetween discrete amplifiers. DRA is based on stimulated Ramanscattering, an inelastic scattering process between photons and opticalphonons in which optical power is transferred from shorter to longerwavelengths.

FIG. 1 shows a typical Raman gain profile (Raman gain spectrum). Themaximum power transfer occurs between wavelengths separated by 13.3 THz(about 100 nm). Two pumping arrangements can be distinguished, as shownin FIG. 2 and FIG. 3.

FIG. 2 shows counter-directional pumping, where the pump light 112propagates in opposite direction to the signal light (signal waves) 111in a transmission fiber 101. In this case, a discrete amplifier 103 anda pump unit 104 are provided in a repeater unit on one side (outputside) of the transmission fiber 101 and coupled to the transmissionfiber 101 through an optical coupler 102. The pump unit 104 comprises aplurality of pump lasers of different wavelengths.

FIG. 3 shows co-directional pumping, where the pump light 113 propagatesin the same direction as the signal light 111. In this case, a pump unit106 is provided in a repeater unit on the other side (input side) of thetransmission fiber 101 and coupled to the transmission fiber 101 throughan optical coupler 105.

In state-of-the art systems, counter-propagation is commonly used inorder to avoid the risk of pump-signal crosstalk.

Employing a plurality of pumps with different wavelengths and suitablepower allows flattening the gain over a wide signal wavelength range asrequired in broadband WDM transmission systems (see Reference 1).

Reference 1

L. Labrunie et al., “1.6 Terabit/s (160×10.66 Gbit/s) unrepeateredtransmission over 321 km using second order pumping distributed Ramanamplification”, OAA 2001 PD3.

FIG. 4 shows this principle schematically with a power spectrum for aC/L-band transmission system with four Raman pumps of differentwavelengths (first-order multiple-wavelength Raman pumping). Pump lightof frequencies f₁, f₂, f₃, and f₄ (f₁>f₂>f₃>f₄) amplifies L-band andC-band signal waves with an appropriate gain over the signal wavelengthrange.

Since the Raman pumping efficiency is polarization sensitive it isnecessary to use depolarized pump light in order to suppresspolarization dependent gain. Depolarization can be achieved bymultiplexing two waves with orthogonal polarization of the same or ofslightly different frequencies f_(p1), f_(p2) given byf_(p1)=f_(p)−δf_(p), f_(p2)=f_(p)+δf_(p), where δf_(p) is up to 0.35THz. Later on, the term “pump” will be used for such pairs ofmultiplexed waves with slightly different frequencies and orthogonalpolarization. As frequency of a depolarized pump the center frequencyf_(p) is used.

The group velocity v_(g), i.e. the speed at which an optical pulsepropagates through a fiber, is wavelength dependent. This phenomenon isknown as group velocity dispersion or chromatic dispersion. FIG. 5 showsschematically the wavelength dependence of the group velocity of astandard single mode fiber (SMF). In the normal dispersion regime, thehigher frequency components travel slower than the lower frequencycomponents (λ<λ_(d), β₂>0). Group velocity dispersion parameter β₂ iswritten as follows by using wavelength λ (ω=2π/λ) and dispersionparameter D.

$\begin{matrix}{\beta_{2} = {{\frac{\mathbb{d}}{\mathbb{d}\omega}\left( \frac{1}{v_{g}} \right)} = {{- \frac{\lambda^{2}}{2\;\pi\; c}}D}}} & (1)\end{matrix}$

Chromatic dispersion causes pulse broadening because the individualspectral components of the pulse propagate at different speeds.

Recently, two new technologies have been introduced to further improvethe performance of Raman amplified transmission systems: second- andthird-order Raman pumping and Raman pump modulation.

Second order Raman pumping uses a second order pumps to amplify firstorder pumps along the transmission fiber. This makes the gainexperienced by the signals more uniform along the fiber, which improvesthe noise figure. It has first been proposed with the first-order pumpcounter-propagating and the second-order pump co-propagating to thesignals (see Reference 2).

Reference 2

K. Rotwitt et al., “Transparent 80 km bi-directionally pumpeddistributed Raman amplifier with second order pumping”, Europeanconference on optical communications 1999, vol. II, pp. 144-145.

Later, second-order Raman pumping with both the first- and second-orderpumps counter-propagating to the signals has been demonstrated (seeReferences 1 and 3).

Reference 3

Y. Hadjar et al., “Quantitative analysis of second order distributedRaman amplification”, OFC 2002 ThB pp. 381-382.

Third-order Raman pumping has also been demonstrated (see Reference 4).

Reference 4

S. B. Papernyi et al., “Third-order cascaded Raman amplification”, OFC2002 postdeadline papers, FB4.

Fludger et al. proposed modulation and temporal separation ofmulti-wavelength Raman pumps as a means to suppress stimulated Ramanscattering and four-wave mixing (FWM) among pumps (see Reference 5).Both effects degrade the performance of broadband WDM transmissionsystems.

Reference 5

C. R. S. Fludger et al., “Novel ultra-broadband high performancedistributed Raman amplifier employing pump modulation”, OFC 2002 WB4.

As Fludger et al. have pointed out, the modulation frequency of theRaman pump light should be in the order of a few 10 MHz to several 100MHz. If the modulation frequency is too low, modulation transfer fromthe pumps to the signals occurs. On the other hand, if it is too high,the pump pulses are dispersed.

Since the Raman effect is practically instantaneous when compared tosignal bit rates, modulation of the pump intensity will cause variationsin the gain experienced by the signals. However, in counter-directionalpumping scheme, the gain is averaged over the effective length of thetransmission fiber such that any pump fluctuations above a few KHz arefiltered. Further, modulation transfer from the pumps to the signals isnegligible for the modulation frequency greater than a few MHz (seeReferences 5 and 6).

Reference 6

C. R. S. Fludger et al., “Pump to signal RIN transfer in Raman fiberamplifiers”, Journal of lightwave technology, Vol. 19, No. 8, pp.1140-1148, August 2001.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide an amplificationsystem that allows further freedom in the control of the lateral signalpower distribution (gain distribution) along the fiber.

The amplification system according to the present invention employsRaman amplification with a plurality of first-order Raman pumps and atleast one second-order Raman pump which amplifies the first-order Ramanpumps. The first- and second-order pumps counter-propagate to signallight in an optical fiber. The amplification system comprises aplurality of light sources and a modulator unit.

In the first aspect of the present invention, the light sources generatepump light of the first- and second-order pumps. The modulator unitmodulates the pump light of the first- and second-order pumps by usingrelative timing of the first- and second-order pumps to optimize lateralsignal power distribution along the optical fiber.

In the second aspect of the present invention, the light sourcesgenerate pump light of the first- and second-order pumps. The modulatorunit modulates the pump light of the first- and second-order pumps byusing relative timing of the first- and second-order pumps to allowflattening lateral signal power distribution along the optical fiber.

In the third aspect of the present invention, the light sources generatepump light of the first- and second-order pumps. The modulator unitmodulates the pump light of the first- and second-order pumps bycontrolling a length of an interaction area in the optical fiber. In theinteraction area pump power of modulated pulses of the second-order pumpoverlap with pump power of modulated pulses of the first-order pumps.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a Raman gain profile.

FIG. 2 shows counter-directional pumping.

FIG. 3 shows co-directional pumping.

FIG. 4 shows principle of first-order multiple-wavelength Raman pumping.

FIG. 5 shows group velocity dispersion in a standard single mode fiber.

FIG. 6 shows the configuration of modulated first- and second-orderRaman pumping.

FIG. 7 shows the power spectrum of modulated first- and second-orderRaman pumping.

FIG. 8 shows modulation of first- and second-order Raman pumps.

FIG. 9 shows impact of modulated second-order Raman pumping.

FIG. 10 shows relationship among pulse parameters of pump i.

FIG. 11 shows the condition of the maximum temporal offset between pump1 and 2.

FIG. 12 shows relationship between the first- and second-order pumppulses at the starting point of overlap.

FIG. 13 shows relationship between the first- and second-order pumppulses at the ending point of overlap.

FIG. 14 shows the configuration of a pump unit employing electricalmodulation.

FIG. 15 shows the configuration of a pump unit employing opticalmodulation.

FIG. 16 shows a pumping concept with reverse third-order pumping.

FIG. 17 shows a pumping concept with forward third-order pumping.

FIG. 18 shows pump pulse shapes for equalizing gain transfer from thesecond-order pump to the first-order pump.

FIG. 19 shows pump pulse shapes in the interaction area.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments according to the present inventionwill be described in detail by referring to the drawings.

In the preferred embodiments the techniques of second-order Ramanpumping and pump modulation are combined. First- and second-order Ramanpumps are both modulated and launched in opposite direction to thesignals. Temporal offsetting of the first- and second-order Raman pumpsprevents an interaction between the pumps close to the fiber end, wherethe launch power of the first-order Raman pumps is still high.

FIG. 6 illustrates this concept for a C/L-band transmission system. Thepump light 202 launched by the pump unit 201 comprises four first-orderRaman pumps and one second-order Raman pump. The power spectrum of thissystem is as shown in FIG. 7. The first-order Raman pumps withfrequencies f₂, f₃, f₄, and f₅ provide gain to the signal channels andthe second-order Raman pump with frequency f₁ pumps the first orderRaman pumps.

The first-order Raman pumps are modulated in phase with a rectangularpulse as shown in FIG. 8, whereas the pulse train of the second-orderpump is out of phase. The horizontal axis in FIG. 8 represents time andthe height of each pulse represents power of the modulated pump. Thetemporal offset of a second-order pump pulse to a first-order pump pulseis determined such that they do not or partially overlap.

As the pumps penetrate deeper into the fiber, their power reduces due toabsorption. However, because of the different group velocities themodulated first- and second-order pump pulses increasingly overlapresulting in a power transfer from the second- to the first-order pumpand as a consequence, a higher gain experienced by the signal fartheraway from the fiber end (launching point of the pumps).

This amplification scheme can be represented by the words “remoteamplification”. “Remote amplification” of the first- and second-orderpumps means that the overlap between the pumps reaches its maximumdeeper in the fiber, thus pushing the power transfer from the second- tofirst-order pump deeper into the fiber. This effect is illustrated inFIG. 9.

In FIG. 9, the second-order pump propagates faster than the first-orderpumps. However, depending on the fiber type and the pump wavelengthallocation, the opposite might be the case. The vertical axis representspower and the horizontal axis represents a distance from another fiberend (the launching point of signals) along the fiber.

The solid lines 211 and 212 indicate the power distribution of a signaland a first-order pump, respectively, without second-order pumping. Thedashed lines 213 and 214 schematically show the impact of pulsedsecond-order pumping on the power distribution of the signal and thefirst-order pump, respectively. Interaction area 215 represents an areawhere the first- and second-order pump pulses overlap and an interactionoccurs between the first- and second-order pumps.

In the case that the second-order pumping is utilized, the launch powerof the first-order pump is reduced such that the resulting gain of thesignals remains constant. The gain is shifted deeper into the fiber suchthat the signals do not drop to such low power levels as in theconventional pumping scheme, which increases the optical-signal to noiseratio.

Next, the relationship between the first- and second-order pump pulsesis discussed. For ease of explanation, a first-order pump and thesecond-order pump are represented by pump 1 and 2, respectively and someparameters are introduced as follows.

-   T_(on,i): time in which pump i is on during one cycle-   T_(off,i): time in which pump i is off during one cycle    T _(i) =T _(on,i) +T _(off,i)-   1/T_(i): modulation frequency of pump i-   Γ_(i)=T_(on,i)/T_(i): duty cycle of pump i-   T_(offset,ij): temporal offset between pump i and j-   v_(gr,i): group velocity of pump i-   D=d(1/v_(gr))dλ: group velocity dispersion (D=17 ps/nm/km for SMF)

The relationship among the parameters T_(i), T_(on,i) and T_(off,i) isillustrated in FIG. 10. If the pulse cycle T₂ of pump 2 is set to matchthe pulse cycle T₁ of pump 1 and the cycle is represented by T, themaximum temporal offset between the pumps is written as follows.T _(offset,12) =T−T _(on,2)=(1−Γ₂)T  (2)

This condition of the maximum temporal offset is shown in FIG. 11. Thegap Δt between the falling edge of the pump 1 pulse and the rising edgeof the following pump 2 pulse is obtained as follows.Δt=T _(offset,12) −T _(on,1)=(1−Γ₁−Γ₂)T  (3)

In a first approximation, the group velocities of the pumps arerepresented by the following expressions with λ₀, λ₁ and λ₂ as areference wavelength, the wavelength of pump 1 and the wavelength ofpump 2, respectively.v _(gr,1)=1/(1/v _(gr,0) +D(λ₁−λ₀))  (4)v _(gr,2)=1/(1/v _(gr,0) +D(λ₂−λ₀))  (5)

In equations (4) and (5), v_(gr,0) represents the group velocity of thelight with the reference wavelength λ₀. The difference of propagationtime from the launching point P_(L) of the pumps to a position P_(z)along the fiber is obtained as follows, provided that L and z representdistances from the launching point of signals to the point P_(L) and theposition P_(z), respectively.(L−z)/v _(gr,1)−(L−z)/v _(gr,2)=(L−z)D(λ₁−λ₂)  (6)

In order to have the overlap of pulses shown in FIG. 11 start at P_(z),Δt has to match the difference of propagation time. From equations (3)and (6), the following equation is obtained.(L−z)D(λ₁−λ₂)=(1−Γ₁−Γ₂)T  (7)

This is rewritten as the following condition for the starting positionP_(z) with κ=T/(D(λ₁−λ₂))L−z=κ(1−Γ₁−Γ₂)  (8)

The interaction time is defined as a time from the start to end of theoverlap. The relationship between the first- and second-order pumppulses at the starting and ending points of the overlap is shown inFIGS. 12 and 13, respectively. As is clear from FIG. 13, the interactiontime is represented by the following expression.T _(on,1) +T _(on,2)=(Γ₁+Γ₂)T  (9)

On the other hand, the interaction time is written by using theinteraction length. The interaction length is the distance between thestarting and ending points of the overlap and corresponds to the lengthof the interaction area 215 in FIG. 9. The interaction time is rewrittenas follows with Δz as the interaction length.Δz/v _(gr,1) −Δz/v _(gr,2) =ΔzD(λ₁−λ₂)  (10)

Assuming that the right side of equation (9) equals the right side ofequation (10), Δz is represented by the following expression.Δz=(Γ₁+Γ₂)T/(D(λ₁−λ₂))=κ(Γ₁+Γ₂)  (11)

From equation (11), it is verified that the interaction length can becontrolled by adjusting the modulation frequency (1/T) and duty cycles(Γ₁ and Γ₂). An optimum overall performance of the system, such asflattening gain distribution along the fiber, is achieved by choosing anappropriate interaction length.

More generally, the modulation frequencies for the first- andsecond-order pumps do not always need to be the same, and the temporaloffset between them is also adjustable. Further, two or moresecond-order pumps with arbitrary temporal offsets can be used.Therefore, the modulation frequencies, the duty cycles and the temporaloffsets of first- and second-order Raman pumps can be adjusted as therelative timing of the pumps in order to achieve an interaction lengthfor an optimum overall performance. Compared to continuous wavesecond-order pumping this method adds freedom in adjusting the signalpower distribution along the fiber.

The pump sources are modulated either directly (electrically) throughthe driving current or optically using an electro-optical modulator.

FIG. 14 shows the configuration of a pump unit employing electricalmodulation. The pump unit shown in FIG. 14 comprises laser driverelectronics 221, laser diodes (light sources) 222 and 223 and amultiplexer 224. The laser driver electronics 221 supplies modulatedcurrent power to the laser diodes 222 and 223.

The laser diodes 222 generate pump lights with wavelengths λ₁ throughλ_(n) by the modulated current for n first-order pumps. The laser diodes223 generate pump lights with wavelengths λ_(n+1) through λ_(n+m) by themodulated current for m second-order pumps. The laser driver electronics221 operates as a modulator unit and controls the relative timing of thefirst- and second-order pumps.

The multiplexer 223 multiplexes and outputs the pump lights from thelaser diodes 222 and 223. The pulse trains 225 and 226 represent typicalpulse shapes of the first- and second-order pumps, respectively.

FIG. 15 shows the configuration of a pump unit employing opticalmodulation. The pump unit shown in FIG. 15 comprises modulator driverelectronics 231, laser diodes 232 and 233, optical modulators 234 and235 and a multiplexer 236. The modulator driver electronics 231 suppliespower to the optical modulators 234 and 235. The laser diodes 232generate lights with wavelengths λ₁ through λ_(n) and the laser diodes233 generate lights with wavelengths λ_(n+1) through λ_(n+m).

The optical modulators 234 modulate the lights from the laser diodes 232to generate pump lights of n first-order pumps. The optical modulators235 modulate the lights from the laser diodes 233 to generate pumplights of m second-order pumps. The modulator driver electronics 231 andthe optical modulators 234 and 235 operate as a modulator unit andcontrol the relative timing of the first- and second-order pumps.

The multiplexer 236 multiplexes and outputs the pump lights from theoptical modulators 234 and 235. The pulse trains 237 and 238 representtypical pulse shapes of the first- and second-order pumps, respectively.

In the configurations shown in FIGS. 14 and 15, the modulation frequencyranges from a few MHz to several hundred MHz, depending on thedifference of the group velocities of the first- and second-order pumps,and the desired interaction length.

In addition, a continuous wave third-order pump light can be launchedeither in forward or in backward direction to compensate for absorptionlosses of the second-order pumps. The third-order pumping is employed toamplify the second-order pump light.

In contrast to modulated first- and second-order reverse pumping withoutthird-order pumping, which is shown in FIG. 6, FIGS. 16 and 17 showpumping concepts with reverse third-order pumping and with forwardthird-order pumping, respectively. The pump unit 241 in FIG. 16generates the first-, second- and third-order pump light propagating inopposite direction to the signal light 111, whereas the pump unit 242 inFIG. 17 generates only the third-order pump light propagating in thesame direction as the signal light 111.

The pump unit 241 can be realized, for example, by adding a laser diodefor the continuous wave third-order pump to one of the configurationsshown in FIGS. 14 and 15. The pump unit 242 also includes such a laserdiode.

According to the third-order pumping scheme, the absorption lossexperienced by the second-order pump is partly compensated by theamplification through the third-order pump. Thus, during the interactionbetween the second- and first-order pump pulses, the gain experienced bythe first-order pump is larger. As a consequence, the signals alsoexperience stronger gain in the interaction area.

Furthermore, the second-order pump light might be modulated such thatthe components overlapping with the first-order pumps deeper within thefiber have a higher launch power. In this case, temporal shapes of thefirst- and second-order pump pulses are controlled as the relativetiming of the pumps. This technique can further help to equalize thegain induced by the first-order pumps along the fiber.

FIG. 18 shows an example of the second-order pump pulse shape in such amodulation concept. The pulse trains 251 and 252 represent typical pulseshapes of the first- and second-order pumps, respectively, at thelaunching point of the pumps (t=0). In the interaction area (t=t₁,t₁>0), the pulse shapes change as shown in FIG. 19 due to the powerabsorption within the fiber.

If the pulse train 252 in FIG. 18 is used instead of the pulse train 226in FIG. 14 or the pulse train 238 in FIG. 15, since the power of thesecond-order pump light increases from the starting point towards theending point of the interaction area, the power transfer from thesecond- to the first-order pump is pushed deeper into the fiber.

The concept of the present invention can be applied not only todistributed Raman amplification but also to discrete fiber Ramanamplifiers (e.g. Raman pumped dispersion compensating fibers or S-bandfiber Raman amplifiers).

1. An amplification system employing Raman amplification with aplurality of first-order Raman pumps and at least one second-order Ramanpump which amplifies the first-order Raman pumps, the first- andsecond-order Raman pumps counter-propagating to signal light in anoptical fiber, the amplification system comprising: a plurality of lightsources generating pump light of the first- and second-order pumps; anda modulator unit modulating the pump light of the first- andsecond-order pumps such that pump power of modulated pulses of the atleast one second-order pump does not overlap with pump power ofmodulated pulses of the first-order pumps at a launching point of thepump light in the optical fiber, and such that pump power of modulatedpulses of the at least one second-order pump overlaps with pump power ofmodulated pulses of the first-order pumps at a position distant from thelaunching point, by using a first timing for the pump light of thefirst-order pumps and a second timing relatively different from thefirst timing for the pump light of the at least one second-order pump toallow flattening lateral signal power distribution along the opticalfiber.
 2. The amplification system according to claim 1, furthercomprising a light source generating pump light of at least onethird-order pump co-propagating with the signal light to amplify thesecond-order pump.
 3. The amplification system according to claim 1,further comprising a light source generating pump light of at least onethird-order pump co-propagating with the second-order pump to amplifythe second-order pump.
 4. The amplification system according to claim 1,wherein the modulator unit controls a temporal shape of modulated pulsesof the second-order pump such that pump power of the modulated pulses ofthe second-order pump overlap with pump power of modulated pulses of thefirst-order pumps.
 5. The amplification system according to claim 4,wherein the modulator unit controls the temporal shape of the pulsessuch that power transfer from the second-order pump to the first-orderpumps is pushed deeper into the optical fiber.
 6. The amplificationsystem according to claim 1, wherein the modulator unit includes driverelectronics controlling the first and the second timing and electricallymodulates the pump light of the first- and second-order pumps throughthe driver electronics.
 7. The amplification system according to claim1, wherein the modulator unit includes optical modulators controllingthe first and the second timing and optically modulates the pump lightof the first- and second-order pumps through the optical modulators. 8.An amplification method employing Raman amplification with a pluralityof first-order Raman pumps and at least one second-order Raman pumpwhich amplifies the first-order Raman pumps, the first- and second-orderRaman pumps counter-propagating to signal light in an optical fiber, theamplification method comprising: generating pump light of the first- andsecond-order Raman pumps; modulating the pump light of the first- andsecond-order Raman pumps by using a first timing for the pump light ofthe first-order pumps and a second timing different from the firsttiming for the light pumped of the at least one second-order pump toallow flattening lateral signal power distribution along the opticalfiber; and launching the pump light of the first- and second-order pumpsin opposite direction to the signal light in the optical fiber, whereinpump power of modulated pulses of the at least one second-order pumpdoes not overlap with pump power of modulated pulses of the first-orderpumps at a launching point of the pump light in the optical fiber, andpump power of modulated pulses of the at least one second-order pumpoverlaps with pump power of modulated pulses of the first-order pumps ata position distant from the launching point.
 9. A Raman amplificationmethod in an optical fiber, comprising: emitting pump light of aplurality of first-order Raman pumps and at least one second-order Ramanpump, first-order pump lights having different time offset between thepump light of the first-order Raman pump and the pump light of the atleast one second-order Raman pump, and different launching powerscorrelated to equalize a gain induced by the plurality of first orderpumps along the fiber, wherein pump light pulses emitted by the at leastone second-order pump do not overlap with pump pulses emitted by thefirst-order pumps at a launching point of the pump light in the opticalfiber, and pump light pulses emitted by the at least one second-orderpump overlap with pump light pulses of the first-order pumps at aposition distant from the launching point.